Method of Hardening Surface of Metallic Part, Piston, Cylinder Head, and Cylinder Block Each Produced Using the Surface-Hardening Method, and Process for Producing the Same

ABSTRACT

With regard to a surface hardening method that a surface of an engine member is coated with an alloy layer having high abrasive resistance so as to improve life of the member and a production method of a piston or the like that abrasive resistance of the member is improved by the hardening method, a surface of a metal base material is coated uniformly with mixture including metal powder, a binder and solvent so as to form a coat, and the coat is dried and applied thereon with laser or electron beam so as to be sintered and dispersed for forming an alloy layer on a surface of the metal base material and for bonding the alloy layer to the metal base material.

TECHNICAL FIELD

The present invention relates to a surface hardening method for a metalmember of an engine or a driving member in the field of a transport ormachine structure and a production method of the member, the memberrequired abrasive resistance.

BACKGROUND ART

Recently, crude oil becomes heavy, demand for light oil is increased andan oil refining method is changed so that fuel characteristic of lowquality fuel oil is worsened. Then, parts of an internal combustionengine tend to be worn by hard particles, sulfur content, fuel residueincluded in fuel.

For dealing with the above situation, for example, as an art forimproving abrasive resistance of a ring groove of an integral FCD(Ferrum Casting Ductile) piston used mainly in a diesel engine, there iswell known and adopted widely a laser quenching art that the ring grooveis quenched by laser (for example, see the Patent Literature 1), a highfrequency quenching art (for example, see the Patent Literature 2) and achromium plating art.

Patent Literature 1: the Japanese Patent Laid Open Gazette Sho.61-149424

Patent Literature 2: the Japanese Patent Laid Open Gazette Hei. 7-119831

However, the hardness of the quenched part treated by the laserquenching art or the high frequency quenching art is about 600 to 800Hv. The structure of the quenched part is not perfectly uniform andstructures such as martensite, bainite and retained austenite areincluded, whereby the hardness is not uniform.

A top ring groove is susceptible to engine combustion temperature andwhen the ring groove is heated upper than about 150° C., the quenchedstructure is tempered and the hardness is reduced for about 100 to 200Hv.

Furthermore, when the piston is made from cast iron which is corrodedeasily, the ring groove is corroded by sulfur or the like included infuel gas.

Because of the above reasons, especially in an internal combustionengine, when the piston ring groove is worn by use for hours and avertical gap is enlarged by the abrasion, lubricating oil is raisedexcessively by pumping action so that consumption of lubricating oil isincreased. Then, when the abrasion reaches a fixed amount, the pistonmust be exchanged, whereby the maintenance requires cost.

The laser quenching causes many shattered cracks when the quenchingdepth is larger than about 300 μm so as not to be able to perform deepquenching. On the other hand, the high frequency quenching can performquenching with depth not larger than about 800 μm, but causes large heattreatment distortion and requires post processing such as grinding.

With regard to the surface hardening by the chromium plating, theplating part is hard for about 800 to 1000 Hv and has high abrasiveresistance, but the plating is very expensive. Furthermore, platingsolution contains hexavalent chromium and is not preferable inconsideration of influence upon the environment.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

The present invention is provided in consideration of the above problemsand the purpose of the invention is to provide a surface hardeningmethod that a surface of an engine member is coated with an alloy layerhaving high abrasive resistance so as to improve life of the member anda production method of a piston or the like that abrasive resistance ofthe member is improved by the hardening method.

Means for Solving the Problems

With regard to a surface hardening method for a metal member accordingto the present invention, a surface of a metal base material is coateduniformly with mixture including metal powder, a binder and solvent soas to form a coat, and the coat is dried and applied thereon with laseror electron beam so as to be sintered and dispersed for forming an alloylayer on a surface of the metal base material and for bonding the alloylayer to the metal base material.

With regard to the surface hardening method for a metal member accordingto the present invention, the coat is coated with black lead so as topromote sintering and dispersing of the metal powder.

With regard to the surface hardening method for a metal member accordingto the present invention, the metal base material is cast iron basematerial.

With regard to a production method of a piston according to the presentinvention, the surface hardening method for a metal member as set forthin one of claims 1 to 3 is adopted to a ring groove of a piston for aninternal combustion engine.

With regard to a production method of a cylinder head according to thepresent invention, the surface hardening method for a metal member asset forth in one of claims 1 to 3 is adopted to a valve seat of acylinder head for an internal combustion engine.

With regard to a production method of a cylinder block according to thepresent invention, the surface hardening method for a metal member asset forth in one of claims 1 to 3 is adopted to a liner surface of acylinder block for an internal combustion engine.

A ring groove of the piston is hardened by the surface hardening methodfor a metal member as set forth in one of claims 1 to 3.

A valve seat of the cylinder head is hardened by the surface hardeningmethod for a metal member as set forth in one of claims 1 to 3.

A liner surface of a cylinder block according to the present inventionis hardened by the surface hardening method for a metal member as setforth in one of claims 1 to 3.

EFFECT OF THE INVENTION

With regard to a surface hardening method for a metal member accordingto the present invention, a surface of a metal base material is coateduniformly with mixture including metal powder, a binder and solvent soas to form a coat, and the coat is dried and applied thereon with laseror electron beam so as to be sintered and dispersed for forming an alloylayer on a surface of the metal base material and for bonding the alloylayer to the metal base material. Accordingly, for example, MC typecarbide such as MoC or VC is dispersed finely and uniformly on thesurface of the ring groove of the piston of FCD so as to form the alloylayer with hardness of about 1000 to 3000 Hv easily, thereby preventingaggressive abrasion caused by combustion residue such as carbon.

With regard to steel material, the hardness is reduced by tempering atthe temperature not less than 150° C. However, the special carbide hashigh melting point and hardly coheres and becomes bulky at hightemperature. Accordingly, the reduction of hardness of the surface ofthe ring groove following the rise of temperature of the ring groove atthe time of driving an engine is prevented.

Furthermore, ceramic alloy which has higher corrosion resistance thansteel material is formed on the surface so as to prevent corrosioncaused by sulfur or the like.

Radiofrequency quenching according to the conventional method requirestempering and grinding after quenching. On the other hand, according tothe present invention, after cuttingly processing the ring groove, apowder alloy layer is formed uniformly with thickness of 100 to 300 μmso as to omit tempering and post processing, whereby the production costis reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 It is a diagram of production processes according to anembodiment of the present invention.

FIG. 2 It is a side view partially in section of a piston of the same.

FIG. 3 It is an enlarged view of an important part of FIG. 2.

FIG. 4 It is a typical sectional view of a cast-iron cylinder headproduced by a method of the present invention.

FIG. 5 It is an enlarged view of a part Z of FIG. 4.

FIG. 6 It is a drawing of a liner surface of a cylinder block.

DESCRIPTION OF NOTATIONS

-   1 a piston-   3, 4 ring grooves-   8 laser light-   10 laser absorbent-   15 a cylinder head-   19 a valve seat-   20, 21, 22 alloy layers-   25 a cylinder block-   26 a liner surface

THE BEST MODE FOR CARRYING OUT THE INVENTION

Next, explanation will be given on an embodiment of the presentinvention.

FIG. 1 is a diagram of production processes according to an embodimentof the present invention. FIG. 2 is a side view partially in section ofa piston of the same. FIG. 3 is an enlarged view of an important part ofFIG. 2. FIG. 4 is a typical sectional view of a cast-iron cylinder headproduced by a method of the present invention. FIG. 5 is an enlargedview of a part Z of FIG. 4. FIG. 6 is a drawing of a liner surface of acylinder block.

Embodiment 1

Explanation will be given on processes of production of a cast-ironcylinder for an internal combustion engine to which a surface hardeningmethod for a metal member according to the present invention is adoptedreferring to FIGS. 1, 2 and 3.

<Casting and Mechanical Processing Process>

Cast iron liquid is poured into a mold so as to cast a piston 1 of FCD(Ferrum Casting Ductile), and then piston ring grooves are processedcuttingly so as to form ring grooves 3 and 4.

<Compounding and Coating Process>

Firstly, several kinds of metal powder are respectively weighed at afixed weight and are compounded. Next, the alloy powder is mixed by aball mill or the like so as to disperse the metal powder equally. Afixed amount of solvent is added to the alloy powder and mixed so as todisperse the alloy powder and the solvent equally. Then, a binder isadded to them and stirred sufficiently so as to make them into an equalmixture.

Next, the ring grooves 3 and 4 are coated by a spray nozzle (not shown)with the mixture of the alloy powder, resin and solvent obtained as theabove. At the time of coating, the piston 1 is rotated at a fixed speedso as to form a uniform powder alloy layer (thickness of 200 to 400 μm).After forming the powder alloy layer, the layer is let at roomtemperature so as to dry the solvent sufficiently.

As the metal powder, it is preferable to use metal which generates veryhard carbide (MC carbide) such as Cr, V, W, Mo or Ti. However, alumina,TiN, TiO₂, CrN, zirconia, SiC, TiC or the like may be used as metalforming hard alloy so as to form the powder alloy layer. The resin ispreferably alkyd resin, but is not limited thereto.

The solvent may be thinner, toluene, xylene or alcohol such as methanol,ethanol or propanol and is selected suitably in consideration of dryingspeed and safety.

As an embodiment, the process is given that the binder is added later tothe mixture of the metal powder and the solvent. However, metal powdermay alternatively be added to a mixture of solvent and a binder.

The thickness of the powder alloy layer is preferably about 200 to 400μm, but is not limited thereto.

Instead of the mixture powder of several kinds of metal, powder of onlyone kind of metal may alternatively be used. For shortening drying time,a coated thing may be entered in a drying oven or the like so as topromote drying of the solvent.

<Absorbent Coating Process>

Laser absorbent 10 is coated by a spraying nozzle on the powder alloylayer with thickness of about 5 to 15 μm corresponding to laserwavelength. At the time of coating, the piston 1 is rotated at a fixedspeed so as to form a uniform coat. After forming the coat, the coat islet at room temperature so as to dry solvent sufficiently.

In this embodiment, black lead powder diluted with the solvent such asthinner is used as the laser absorbent 10 so as to form a black leadcoat on the powder alloy layer. However, the process is not limitedthereto and coating agent whose principal component is ferric oxide orthe like may alternatively be coated as the absorbent.

The diluent solvent may be thinner, toluene, xylene or alcohol such asmethanol, ethanol or propanol and is selected suitably in considerationof drying speed and safety.

<Alloying Process>

As shown in FIGS. 2 and 3, laser or electron beam is applied withsuitable output and scanning speed on the ring grooves 3 and 4 so as tosinter or melt the powder alloy layer, whereby an alloy layer 20 withthickness of about 150 to 350 μm is formed on the ring grooves 3 and 4.

The laser may be CO₂ laser, YAG laser, semiconductor laser or the like.The electron beam having higher energy than the laser also may be used.

Accordingly, by coating the laser absorbent 10 (black lead coat) at theformer process, laser light 8 is absorbed efficiently to the powderalloy layer so as to heat the powder alloy layer. Then, sintering,melting and dispersion to the metal base material of the piston and thelike (cast iron base material) is promoted in the powder alloy layer andthe interface between the powder alloy layer and the base material,whereby the alloy layer which is stable and has abrasive resistance isgenerated on the surface of the metal base material, and the alloy layeris bonded to the metal base material (cast iron base material).

In this embodiment, a member made from FCD (Ferrum Casting Ductile) isused as the metal base material. However, the metal base material is notlimited thereto and aluminum alloy or the like may be used.

<Finishing Process>

With regard to the ring grooves 3 and 4 of the piston 1, the surface ofthe alloy layer 20 is grinded as occasion demands.

According to the processes, the piston having the alloy layer on thering grooves 3 and 4 is produced.

Next, explanation will be given on a concrete embodiment of theproduction of the piston 1.

Firstly, Mo (molybdenum) powder is weighed and added to toluene which isthe solvent and mixed with a ball mill. Subsequently, a fixed amount ofphthalic resin is added and the whole of them is stirred so as to be auniform mixture.

Next, the recess of the ring groove 3 is coated by the spray nozzle withthe mixture uniformly while rotating the piston of FCD along itsperipheral direction so as to form a powder alloy layer with thicknessof about 300 μm. After forming the powder alloy layer, the layer is letat room temperature so as to dry the solvent sufficiently.

Next, the powder alloy layer is coated by the spray nozzle with thelaser absorbent 10 with thickness of about 10 μm. At the time ofcoating, the piston is rotated at a fixed speed along its peripheraldirection so as to form the uniform coat. After forming the coat, thecoat is let at room temperature so as to dry the solvent sufficiently.

As shown in FIGS. 2 and 3, the laser light 8 of CO₂ laser is applied onthe ring groove 3. For example, in the case of forming the alloy layer20 on the upper side surface of the ring groove 3, the laser light 8 iscondensed by a condenser lens 5 and then applied on the upper surfacewith an incidence angle α by a reflecting mirror 6 while rotating thepiston 1, whereby the alloy layer 20 is formed on the whole perimeter ofthe groove. In the case of forming the alloy layer 20 on the lower sidesurface of the ring groove 3, either the laser light 8 may be appliedwhile reversing the piston 1 or the laser light 8 may be applied whilechanging the incidence angle α of the laser light 8.

In this embodiment, especially in the case of forming the alloy layer 20on the both side surfaces of the ring groove 3, the laser light 8 isapplied with an incidence angle α as shown in FIG. 2 while an edge part12 is masked by a masking plate 9 coated with black lead similar to thelaser absorbent 10 with which the powder alloy layer on the groove iscoated. Accordingly, the alloy layer 20 can be formed only in anabrasion area 11 without adjusting the thickness of the laser absorbent10 at the edge part 12 by peeling it, whereby the edge part 12 is notmelted so as to prevent failure, such as a crack, caused by melting.

The piston that the alloy layer 20 is formed in the abrasion area 11 ofthe ring groove 3 is produced as mentioned above.

<Abrasive Resistance Evaluation Method>

Under the production condition similar to the above processes, the alloylayer 20 is formed on a test piece of ferrum casting ductile (30×100(mm)), and the hardness of the surface is measured by a Vickers hardnessmeter, whereby abrasive resistance evaluation is performed. With regardto the test piece on which the alloy layer 20 is formed with Mo(molybdenum) as the above, Vickers hardness is about 2000 Hv. It isconfirmed that the alloy layer 20 is about sextuple harder than ferrumcasting ductile which is the base material (whose Vickers hardness is300 to 350 Hv).

<Alloy Layer Analytical Method>

With regard to the test piece, the interface between the powder alloylayer 20 and the test piece which is the cast iron base material isanalyzed by an EDX (energy dispersive fluorescent X-ray analyzer) so asto confirm that Mo (molybdenum) is distributed gradiently, that is, thepowder alloy layer 20 is bonded to the cast iron base material. An X-raydiffraction device is used so as to confirm that Mo is linked to C so asto generate MoC (molybdenum carbide) which is an example of ceramicalloy having high hardness and high melting point.

According to the processes, binder and solvent are mixed, the surface ofthe metal base material is coated with the mixture uniformly so as toform the coat (powder alloy layer), and the coat is dried and issintered, melted and dispersed by applying the laser 8 or electron beam,whereby the powder alloy layer 20 is generated on the surface of themetal base material and is bonded to the metal base material. Byadopting the surface hardening method according to the present inventionto the production of a metal member, for example, MC type carbide suchas MoC or VC is dispersed finely and uniformly on the surface of thering groove of the piston of FCD so as to form the alloy layer withhardness of about 1000 to 3000 Hv easily. Accordingly, abrasiveresistance or heat resistance is improved widely so as to preventaggressive abrasion caused by combustion residue such as carbon.

When expensive alloy is used, the use is limited to the necessary part,whereby the amount of the use is small economically.

With regard to steel material, the hardness is reduced by tempering atthe temperature not less than 150° C. However, the special carbide suchas MoC has high melting point and hardly coheres and becomes bulky athigh temperature. Accordingly, the reduction of hardness of the surfaceof the ring groove following the rise of temperature of the ring grooves3 and 4 at the time of driving an engine is prevented.

Furthermore, the above-mentioned ceramic alloy which has highercorrosion resistance than steel material is formed on the surface so asto prevent corrosion caused by sulfur or the like.

Radiofrequency quenching according to the conventional method requirestempering and grinding after quenching. On the other hand, according tothe present invention, after cuttingly processing the ring groove 3, thepowder alloy layer 20 is formed uniformly with thickness of 100 to 300μm so as to omit tempering and post processing, whereby the productioncost is reduced.

Embodiment 2

Next, explanation will be given on an embodiment that the surfacehardening method according to the present invention is adopted to a castiron cylinder head for an internal combustion engine referring to FIGS.4 and 5.

A cylinder head 15 is produced under the condition similar to theembodiment 1 except for using mixture powder of V (vanadium) and C asthe metal powder.

As shown in FIG. 5, in the cylinder head 15, an intake valve 16 and anexhaust valve 17 are vertically slidably supported through a valvesystem 18. An alloy layer 21 is formed with thickness of 150 to 350 μmon a valve seat 19, which is a sliding part of the cylinder head 15,according to the production method of the present invention. At theinterface between the alloy layer 21 and the cylinder head 15 which isthe cast iron base material, V is distributed gradiently and VC(vanadium carbide) is generated. Similarly to MoC of the embodiment 1,VC is known as ceramic alloy having high hardness, high abrasiveresistance and high heat resistance (with Vickers hardness of about 2500to 2800 Hv), and is very effective for improving the abrasive resistanceof the sliding part. As the above, the metal powder is heated and meltedby the laser light 8 and sintering so that dispersing is promoted insidethe alloy layer 21 and at the interface between the alloy layer 21 andthe cylinder head 15 which is the base material, whereby the valveseat-integral cylinder head 15 is produced which has the alloy layer 21having the abrasive resistance on the surface of the valve seat 19.

The steel cylinder head 15 produced by the above processes isconstructed that the valve seat 19 and the cast iron base material(cylinder head 11) are integral with each other so as to improve bondingstrength of a boundary between the cylinder head 11 which is the castiron base material and the alloy layer 21. Namely, the bonding strengthis higher than that of a cast iron cylinder head of the conventionalvalve seat-engaging type.

Next, explanation will be given on an embodiment that the surfacehardening method according to the present invention is adopted to aliner surface of a cast iron cylinder block for an internal combustionengine referring to FIG. 6.

A cylinder block 25 is produced under the condition similar to theembodiment 1 except for using mixture powder of W (tungsten) and C asthe metal powder.

As shown in FIG. 6, an alloy layer 22 is formed with thickness of 150 to350 μm on a liner surface 26, which is a sliding part of the cylinderblock 25, according to the production method of the present invention.At the interface between the alloy layer 22 and the liner surface 26 ofthe cylinder block 25 which is the cast iron base material, W isdistributed gradiently and WC (tungsten carbide) is generated. Similarlyto MoC of the embodiment 1 and VC of the embodiment 2, WC is known asceramic alloy having high hardness, high abrasive resistance and highheat resistance (with Vickers hardness of about 2600 to 2800 Hv), and isvery effective for improving the abrasive resistance. As the above, themetal powder is heated and melted by the laser light 8 and sintering sothat dispersing is promoted inside the alloy layer 22 and at theinterface between the alloy layer 22 and the liner surface 26 of thecylinder block 25 which is the base material, whereby the cast ironcylinder block 25 is produced which has the alloy layer 22 having theabrasive resistance on the liner surface 31.

This embodiment explains the method for forming an alloy layer havingabrasive resistance and heat resistance in a metal member for aninternal combustion engine. However, an alloy layer also can be formedon a surface of a member for any other use application requiringabrasive resistance so as to improve the abrasive resistance.

INDUSTRIAL APPLICABILITY

The present invention can be adopted widely to not only a metal memberfor an internal combustion engine but also a surface of a memberrequiring abrasive resistance.

1. A surface hardening method for a metal member characterized in that:a surface of a metal base material is coated uniformly with mixtureincluding metal powder, a binder and solvent so as to form a coat; andthe coat is dried and applied thereon with laser or electron beam so asto be sintered and dispersed for forming an alloy layer on a surface ofthe metal base material and for bonding the alloy layer to the metalbase material.
 2. The surface hardening method for a metal member as setforth in claim 1, wherein the coat is coated with black lead so as topromote sintering and dispersing of the metal powder.
 3. The surfacehardening method for a metal member as set forth in claim 1, wherein themetal base material is cast iron base material.
 4. A production methodfor a piston characterized in that the surface hardening method for ametal member as set forth in claim 1 is adopted to a ring groove of apiston for an internal combustion engine.
 5. A production method for acylinder head characterized in that the surface hardening method for ametal member as set forth in claim 1 is adopted to a valve seat of acylinder head for an internal combustion engine.
 6. A production methodfor a cylinder block characterized in that the surface hardening methodfor a metal member as set forth in claim 1 is adopted to a liner surfaceof a cylinder block for an internal combustion engine.
 7. A piston foran internal combustion engine characterized in that a ring groove of thepiston is hardened by the surface hardening method for a metal member asset forth in claim
 1. 8. A cylinder head for an internal combustionengine characterized in that a valve seat of the cylinder head ishardened by the surface hardening method for a metal member as set forthin claim
 1. 9. A cylinder block for an internal combustion enginecharacterized in that a liner surface of the cylinder block is hardenedby the surface hardening method for a metal member as set forth in claim1.